1. Field of the Invention
The invention relates to downhole inspection systems and, more particularly to a fiberoptic cable assembly for use within reeled tubing. The invention may be used in connection with the maintenance and servicing of oil, gas, geothermal and injection wells.
2. History of the Prior Art
In the drilling and production of oil and gas wells, it is often necessary to obtain at the surface information concerning conditions within the borehole. For example, tools and other objects may become lodged in the borehole during the drilling of a well. Such objects must be retrieved before drilling can continue. When the removal of foreign objects from a borehole is undertaken, known as "fishing", it is highly desirable to know the size, position, and shape of the obstructing object in order to select the proper fishing tool to grasp the object and remove it from the borehole. Such information is very difficult to obtain because of the hostile downhole environment within a borehole filled with opaque drilling fluid.
In the operation and/or periodic maintenance of producing injection wells, it is also frequently necessary to obtain information about the construction and/or operating condition of production equipment located downhole. For example, detection of the onset of corrosion damage to well tubing or casing within a borehole enables the application of anti-corrosive treatments to the well. Early treatment of corrosive well conditions prevents the highly expensive and dangerous replacement of corrosion damaged well production components. Other maintenance operations in a production well environment, such as replacement of various flow control valves or the inspection of the location and condition of casing perforations, make it highly desirable for an operator located at the surface to obtain accurate, real-time information about downhole conditions. The presence of production fluids in the well renders accurate inspection very difficult.
Various techniques have been proposed for obtaining at the surface information about the conditions within a borehole. One approach has been to lower an inspection device, such as a acoustic inspection sensor or an optical inspection sensor, attached to the end of reeled tubing as shown in U.S. Pat. No. 4,938,060 to Sizer, et al. In this system, a quantity of optically clear and/or acoustically homogeneous fluid is directed down the reeled tubing and exits at the lower end thereof in the region of the sensor. The exiting fluid forms a narrow zone of optically transparent and/or acoustically homogenous fluid within an inspection zone inside of the borehole to allow the inspection sensor to accurately observe conditions within the borehole. This system is a substantial improvement over prior art downhole inspection techniques. However, certain specialized operating conditions may occur within the borehole during the inspection process which require certain modifications to the system taught in the above referenced Sizer et al patent.
In certain situations it may desirable to perform additional operations downhole while at the same time conduct an optical or acoustic inspection by means of the optically transparent or acoustically homogenous fluid being sent down the reeled tubing. For example, it may be desirable to circulate a cooling fluid through the electronic equipment with which the inspection is being performed in order to maintain a proper operating temperature of that equipment. Additionally, it may be desirable to inject a corrosion inhibiting fluid at the same time the inspection operation is going on in order to protect the inspection equipment from a highly caustic and corrosive environment within the borehole. The configuration of the system of the present invention enables these additional features and functions.
An additional problem associated with the operation of an inspection system of the type shown in the above referenced Sizer et al patent is that of bringing information gathered during the inspection process up the borehole to the surface while at the same time sending control signals downhole to control the inspection sensor such as a television camera. One form of cable used for the transmission of television signals is that of fiberoptics. While such cables and means for their interconnection are well known, as in U.S. Pat. Nos. 4,924,870 and 4,964,685, respectively, the use of fiberoptic cables in a downhole environment is relatively specialized. Moreover, even though it is also known to thread a cable through a length of reeled tubing, as shown in U.S. Pat. No. 4,971,147, the adaptation of a fiberoptic downhole cabling system requires specialized characteristics and features. For example, the cabling used for this purpose must be furnished in extremely long lengths and, as a result, be relatively strong in order to support extremely long lengths of its own weight extending down the borehole. Further, in certain applications the fluids within the borehole are also highly corrosive and a means to protect such cabling from the deleterious affects of a highly corrosive environment is not only desirable but in some cases essential in order to enable the operation.
It would be a major improvement in downhole inspection systems in which an optically clear or acoustically homogenous fluid is injected in the borehole in the zone where inspection is desired to enable the simultaneous injection of a second fluid to serve a different function and to provide an improved cabling arrangement to use in connection with the reeled tubing to carry both power and information and control signals between the surface and the downhole equipment.